The present invention relates to a particulate trap for trapping and removing particulates such as carbon contained in diesel engine exhausts.
Exhaust gases from automobiles are a major cause of air pollution. It is therefore of extreme importance to develop a technique for removing noxious components in exhausts.
In particular, it is most urgently required to develop a technique for removing particulates components in diesel engine emissions, which are mainly made up of NOx and carbon.
To remove such noxious components in exhausts, Unexamined Japanese Patent Publication 58-51235 proposes to provide an exhaust trap in an exhaust line to trap any noxious components and remove them by after-treatment. Other conventional exhaust purifying means include various improvements in the engine itself such as exhaust gas recirculation (EGR) systems and improved fuel injection systems. But none of them can be a decisive solution. As of today, after-treatment methods are considered more practical as exhaust purifying means. Rigorous efforts are now being made to develop after-treatment type exhaust purifying systems.
It is required that particulate traps for trapping particulates contained in diesel engine emissions satisfy all of the following requirements.
1) Particulate trapping capability
A particulate trap has to be capable of trapping particulate with such high efficiency that exhausts are purified to a satisfactory degree. It is considered necessary to trap an average of at least 60% of the particulates contained in diesel engine emissions, though the amount of particulates contained in exhausts depends upon the displacement of the diesel engine and the load applied.
Airborne particulates 2 .mu.m or less in diameter can enter human alveoli and reportedly trigger lung cancer. It is thus necessary that particulate traps be capable of efficiently trapping such small suspended particulates.
2) Pressure drop
The particulate trap has to be capable of keeping the pressure drop in exhaust gases to a minimum. If it causes a large pressure drop, a back pressure will act on the engine, aggravating the fuel economy. Thus, it is necessary that the particulate trap have a small initial exhaust pressure drop (when no particulates are trapped). As it traps particulates, its filter tends to be clogged with the trapped particulates, so that the pressure drop will increase gradually. It is necessary to keep the pressure drop to a minimum even after it has trapped a large amount of particulates.
3) Regeneration
The third requirement is that the trap can be regenerated at low energy cost. This is because the particulate trap has to be regenerated or recycled many times for repeated use by burning trapped particulates. An electric heater or a light oil burner is considered as a feasible means for burning off particulates.
4) Durability
Fourthly, the trap has to be sufficiently durable. It has to be highly resistant to corrosion when exposed to hot exhaust gases and to heat shocks produced while burning particulates.
5) Integration with a catalytic converter
Further, it is necessary to provide a catalytic converter integral with the trap. In order to remove noxious gas components in exhausts, a catalytic converter carrying a noxious gas removing catalyst may be provided in an engine exhaust line. If it is desired to further provide a separate particulate trap in the same exhaust line, there may be no available mounting space in the exhaust line. Also, the cost for separately providing such two different kinds of after-treatment type exhaust purifying systems tends to be rather high.
One of the existing filter element materials that satisfy the above-listed requirements is a wall-flow type, honeycomb porous member made of cordierite ceramics. It is considered the most practical.
But this filter has several problems. One problem is that particulates tend to collect locally. Another problem is that, because of low thermal conductivity of cordierite ceramic, this filter tends to develop heat spots during regeneration. As a result, the filter may melt or crack due to thermal stress. Such a filter is not durable enough. Unexamined Japanese Patent Publication 4-265411 proposes to uniformly heat a ceramic foam filter by optimally arranging regenerating heaters. But in this arrangement, the filter supporting portion is relatively small, so that its durability is not so high as a honeycomb porous filter when subjected to vibration or exhaust pressure. A ceramic fiber trap made by forming ceramic fibers into a candle shape is gaining much attention these days. But this trap is not sufficiently durable either, because the fibers forming the trap tend to be broken due to reduced strength when exposed to high-temperature exhaust gases.
Metal traps (as disclosed in Unexamined Japanese Patent Publications 6-257422, 6-294313, 7-731 and 7-51522) are now considered more promising, because they are high in thermal conductivity, less likely to develop heat spots and cracks and highly corrosion-resistant in high-temperature exhausts.
We will discuss problems of conventional metal traps in connection with the abovementioned requirements 1)-5).
Conventional metal traps basically satisfy requirements 1) and 3). But as to the capacity of trapping suspended particulates 2 .mu.m or less in diameter in requirement 1), higher performance is desired.
As to requirement 2), conventional metal traps can cause a marked pressure drop after trapping particulates. If an especially low engine back pressure is required, these traps will be unsatisfactory. In order to minimize the pressure loss even after the filter element has trapped a large amount of particulates, the filter element has to have a large surface area (filtering area). But in order to increase the filtering area of a conventional metal trap filter element, it is necessary to use an extremely large trap.
In connection with requirement 4) the filter element of a conventional metal trap is deformed microscopically due to the pressure of the introduced exhausts, and can be destroyed due to stress resulting from the microscopic deformation. Also, since the trap is mounted in the exhaust line, its filter element is vibrated together with the trap, which may result in the destruction of the filter element. These problems are observed in harsh endurance tests.
As to requirement 5), it is sometimes necessary to integrally provide a catalytic converter on a conventional metal trap. For example, a catalyst may be integrally carried on a wall-flow type, honeycomb porous member made of a cordierite ceramic, which was originally developed as a DPF. In such a case, it may be difficult to heat the catalyst to a temperature at which it acts because the honeycomb porous member, having a large heat capacity, is slow to heat up.
An object of the present invention is to provide a particulate trap which is free of all the above problems and which satisfies all the requirements 1)-5).